1. Field of the Invention
The present invention relates to a power semiconductor module with a housing, at least one electrically insulating substrate, and flush-contact type terminal elements having an improved service life. More specifically, the present invention relates to a power semiconductor module. Terminal elements enable an external load or auxiliary contact and include at least one discontinuous contact surface capable of distributing operational strain and stress thereby minimizing element failure.
2. Description of the Related Art
The present invention relates to power semiconductor modules comprised of a housing with base plate, or adapted for mounting on a heat sink, and at least one electrically insulating substrate arranged therein. The substrate is comprised of an insulating body and a plurality of metal connecting tracks, electrically insulated from each other, with the power semiconductor components being located on the connecting tracks and connected to these connecting tracks via appropriate circuitry. On the substrate's lower face it advantageously has a metal layer, comparable to the connecting tracks. Furthermore such power semiconductor modules have terminal elements for external load and auxiliary contacts. Some power semiconductor modules also have connecting elements for connections on the inside of the power semiconductor module. According to the state of the art, full surface contact flush type connections, mainly in the form of soldered joints, but also glued joints, are known.
The related also art involves power semiconductor modules similar to those noted in German patent disclosure DE 39 37 045 A1 or non prior published DE 10 2004 019 568 A1. German patent disclosure DE 39 37 045 A1 discloses a power semiconductor module in the form of a half bridge circuit arrangement.
As discussed in the reference, the external load terminal elements for the two direct current connections and for the alternating current connection in this particular power semiconductor module are provided in the form of two-dimensional metal pieces. These metal pieces in the form of connecting bands are arranged in close proximity to each other in order to reduce parasitic inductivity on the inside of the module. Due to the band-type embodiment for reducing parasitic inductivity, these connecting bands have several “foot points” to provide contact with the strip conductors of the substrate. Detrimentally to compact circuit design, these foot points are arranged at a distance with regard to each other that is greater than their lateral dimension.
German patent disclosure DE 10 2004 019 568 A1 shows a similar embodiment of the terminal elements of a power semiconductor module. Here, the surrounding areas of the foot points of the terminal elements have a plurality of indentations in the printed circuit board of the substrate in order to accommodate excess amounts of solder or sloppy solder results. In this disclosure, the foot point of the terminal element is a two-dimensional part of the terminal element in the form of a metal piece with the face being arranged in parallel with regard to the substrate.
Unfortunately, according to the above publications that embody the related art, the substrates of such power semiconductors are insulating substrates comprised of an insulating body as carrier material and for purposes of electric insulation with regard to a base plate or heat sink.
According to the related art, this insulating body is comprised of industrial ceramics such as aluminum oxide or aluminum nitrite. On this insulating body, on the “first-main” face pointing away from the base plate or heat sink, a plurality of connecting tracks is arranged, electrically insulated from each other. The power semiconductor components in turn are arranged on them.
In most cases the insulating body, on the “second-main” face facing the base plate or the heat sink, also has a metal layer of the same material and same thickness as the connecting tracks on the first-main face. Usually, however, this layer is not structured in itself since it is used for the soldered connection to the base plate. The connecting tracks as well as the metal layer of the second main face preferably are comprised of copper that is applied based on the direct copper bonding DCB method.
Connecting elements are arranged to connect the strip conductors to the power semiconductor components with the connecting elements, and are comprised of metal pieces or wire bond connections according to the related art.
Furthermore, terminal elements, also in the form of metal pieces, are arranged in a flush manner on these connecting tracks to provide external electrical connections for the power semiconductor module. Preferably, the terminal elements, as well as the connecting elements in the form of metal pieces, are made of copper since copper has especially advantageous electric properties. In addition, these metal pieces connecting elements and terminal elements may have a tin or silver layer on their faces to improve the soldering properties.
As a substantial detriment in the related art, flush mounting connections between the connecting tracks of the substrate and terminal or connecting elements are subject to mechanical stress. This mechanical stress originates in the temperature stress of the power semiconductor module. This type of power semiconductor module is subject to operating temperatures that range between −40° C. and +90° C., for example.
The material used, in this case copper, determines the thermal coefficient of expansion of both the terminal and connecting elements. However, the characteristics of the insulating body substantively determine the thermal coefficient of the substrate. The thermal stress of the power semiconductor module and the substrate's thermal coefficient of expansion are considerably lower when compared to the thermal coefficient of expansion for the terminal and connecting elements. This substantive difference causes mechanical stress at the connecting point between substrate and the terminal and/or connecting element and is a principal cause of connection failure resulting in breakage of the flush mounting connection.
Other examples of related art flush terminal and/or connecting elements are disclosed in German publications DE 35 05 086 A1, DE 44 46 527 A1, and DE 101 03 084 A1. Unfortunately, upon review these publications fail to provide any additional information with regard to the embodiment of such a flush terminal or connecting element in a manner provided by the present invention. These references only depict differing embodiments of the course and the foot points of the terminal elements.
What is not appreciated by the prior art is the need for an improved flush connection mounting design with increased reliability and service life without expanding a mount area, and while maintaining the scope and spirit of the presently known flush mounting manufacturing techniques of soldering, gluing, and others know to those of skill in the semiconductor module manufacturing arts.
Accordingly, there is a need for an improved design that accommodates the mechanical and thermal stresses associated with flush connections in power semiconductor modules in a convenient manner that does not detrimentally require substantive change in presently known manufacturing method.